Dawn

Dawn is a NASA probe designed to explore the dwarf planet Ceres and the second largest asteroid, Vesta. Dawn
was originally scheduled for launch in June 2006, was cancelled in March
2006 for budgetary reasons, but was then reinstated and finally launched
on September 27, 2007, by a Delta II rocket
from Cape Canaveral Air Force Station.

Dawn arrived at Vesta and went into orbit around it on July 15, 2011. It
remained around Vesta for more than a year before firing its ion engines
just over a year later and setting course for a rendezvous with Ceres in
2015. A web video celebrating Dawn's "greatest hits" at Vesta is available
at this NASA
webpage.

The probe's instruments are designed to measure the exact mass, shape, and
spin of Vesta and Ceres from orbits 100 to 800 km high, record their magnetization
and composition, use gravity and magnetic data to determine the size of
any metallic core, and use infrared and gamma-ray spectroscopy to search
for water-bearing minerals. Flybys of more than a dozen other asteroids
are planned along the way.

Dawn carries three science instruments: a visible camera, a visible and
infrared mapping spectrometer, and a gamma-ray and neutron spectrometer.
In addition to these instruments, radiometric and optical navigation data
will provide data relating to the gravity field and thus bulk properties
and internal structure of the two bodies.

Dawn's ion propulsion

Dawn is equipped with three 30-cm-diameter (12-inch) ion thrust units (called XIPS), each movable in two axes to allow for
migration of the spacecraft's center of mass during the mission. This also
allows the attitude control system
to use the ion thrusters to help control spacecraft attitude.

A total of three units are needed to provide enough thruster lifetime to
complete the mission and still have adequate reserve. However, only one
thruster will be operate at any given time. Dawn will use ion propulsion
for years at a time, with interruptions of only a few hours each week to
turn to point its antenna to Earth. Total thrust time through the mission
will be about 2,100 days, well in excess of Deep
Space 1's 678 days of ion propulsion operation.

At maximum thrust, each engine produces a total of 91 millinewtons –
about the amount of force involved in holding a single piece of notebook
paper in your hand. At maximum throttle, it would take Dawn's system four
days to accelerate from 0 to 60 miles per hour. As slight as that might
seem, over the course of the mission the total change in velocity from ion
propulsion will be comparable to the push provided by the Delta II rocket
that carried it into space – all nine solid-fuel boosters, plus the
Delta's first, second and third stages. This is because the ion propulsion
system will operate for thousands of days, instead of the minutes during
which the Delta performs.